constify some blockvector APIs

Generally, the blockvector ought to be readonly.  So, this patch makes
the blockvector const in the symtab, and also changes various
blockvector APIs to be const.

This patch has a couple of spots that cast away const.  I consider
these to be ok because they occur in mdebugread and are used while
constructing the blockvector.  I have added comments at these spots.

2014-06-18  Tom Tromey  <tromey@redhat.com>

	* symtab.h (struct symtab) <blockvector>: Now const.
	* ada-lang.c (ada_add_global_exceptions): Update.
	* buildsym.c (augment_type_symtab): Update.
	* dwarf2read.c (dw2_lookup_symbol): Update.
	* jit.c (finalize_symtab): Update.
	* jv-lang.c (add_class_symtab_symbol): Update.
	* mdebugread.c (parse_symbol, add_block, sort_blocks, new_symtab):
	Update.
	* objfiles.c (objfile_relocate1): Update.
	* psymtab.c (lookup_symbol_aux_psymtabs)
	(maintenance_check_psymtabs): Update.
	* python/py-symtab.c (stpy_global_block, stpy_static_block):
	Update.
	* spu-tdep.c (spu_catch_start): Update.
	* symmisc.c (dump_symtab_1): Update.
	* symtab.c (lookup_global_symbol_from_objfile)
	(lookup_symbol_aux_objfile, lookup_symbol_aux_quick)
	(basic_lookup_transparent_type_quick)
	(basic_lookup_transparent_type, find_pc_sect_symtab)
	(find_pc_sect_line, search_symbols): Update.
	* block.c (find_block_in_blockvector): Make "bl" const.
	(blockvector_for_pc_sect, blockvector_for_pc): Make return type
	const.
	(blockvector_contains_pc): Make "bv" const.
	(block_for_pc_sect): Update.
	* block.h (blockvector_for_pc, blockvector_for_pc_sect)
	(blockvector_contains_pc): Update.
	* breakpoint.c (resolve_sal_pc): Update.
	* inline-frame.c (block_starting_point_at): Update.

gdb/symtab.h

@@ -882,7 +882,7 @@ struct symtab
      between different symtabs (and normally is for all the symtabs
      in a given compilation unit).  */
 
-  struct blockvector *blockvector;
+  const struct blockvector *blockvector;
 
   /* Table mapping core addresses to line numbers for this file.
      Can be NULL if none.  Never shared between different symtabs.  */